The heavy-duty corrosion-resistant electrical conduit systems presently being used are typically comprised of coated metal electrical conduits and fittings. Present corrosion-resistant electrical conduit is generally fabricated by coating a standard pipe (the terms “pipe” and “conduit” are referred to interchangeably herein) with polymeric materials. The interior coating of the pipe is applied using a long spraying wand inserted inside the conduit. This method takes a significant amount of time and the resultant thickness of the polymer coating is inconsistent and, hence, requires more material than might otherwise be necessary to ensure adequate coverage. Additionally, the varying thickness of the interior coating reduces the conduit cross-sectional area and increases pulling force requirements for wires and cables.
The surfaces of the corrosion resistant conduit include two polymeric coats. The first and innermost surface coating is applied in a manner similar to the interior coating, while the second and outermost coating is applied by dipping the pipe into a heated organosol bath, then rotating the pipe until coated. For end product use, the finished conduits are then connected and fastened with other components in the conduit system using threaded ends or via non-threaded methods. Fittings, such as couplers and conduit bodies, are basic metal components, which also achieve corrosion resistance through polymeric coatings using an application process similar to the process used to coat the conduit. Connecting corrosion-resistant conduit and conduit fittings is subsequently a careful and time-consuming process, due to the tedious nature of maintaining the coatings through the mechanical actions of the conduit system assembly.
In certain environments, corrosion resistance is a significant limiting factor in determining the lifetime of electrical supply infrastructure. Currently, corrosion-resistant conduit systems include PVC-only conduit, fiberglass composite or traditional rigid metallic conduit over-coated with polymeric coatings. Plastic coatings prevent salts, cleaning products, and/or process chemicals, etc., from oxidizing the metallic components of the conduit system that would in turn lead to exposure of the conductor cables, connectors and associated components. This degree of corrosion also adversely affects electrical safety due to reduced electrical continuity of the electrical system, including grounding, and also may allow foreign objects to enter the conduit and directly impact conductors, which also increases the likelihood of faults.
The National Electrical Code® (NEC®) recognizes several types of conductors that are permitted to be used as equipment grounding conductors, including rigid metal conduit (such as steel, copper and aluminum). For example, steel (or aluminum) conduit used in secondary power distribution systems is designed in such a way that the steel conduit does not carry any appreciable electric current under normal operating conditions. However, under certain fault conditions, the metallic conduit, acting as an equipment grounding conductor, will carry most of the return fault current, or, in some cases, the conduit will be the only return path of the fault current to the source. NEC® Article 250 requires that the metal parts in an electrical system must form an effective low impedance path to ground in order to safely conduct any fault current and facilitate the operation of overcurrent devices protecting the enclosed circuit conductors. UL 514c describes non-metallic conduit, for different applications.
While threaded joints are preferred for rigid metal conduit (“RMC”) and intermediate metal conduit (“IMC”)—thick wall types of conduits—for thin walled conduit, such as electrical metallic tubing (“EMT”), there exists set screw and compression types of connections. Traditionally, the joints that formed the interfaces between conduit sections and between conduits terminated in conduit bodies or boxes were both electrical and mechanical. That is, for set-screw connected EMT, the set-screw provided both the electrical continuity and the mechanical fixation of the conduit system components. With thinner polymer coated conduit, there is not an acceptable method for electrical and mechanical assembly of the system components, as the thin walled metallic tube cannot be effectively threaded. However, the outer polymeric layer of the coated conduit may be dimensionally controlled such that a mechanical connection method may be utilized on the outer surface of the conduit. An ability to create an outer polymer layer that is stiffer or more abrasion resistant also allows the outer polymer layer to be used as a mechanical connection possibility.
The field installation of electrical conduit requires conduit that is capable of being field bent to form a curved path for cables and conductors. In addition, coated conduit does not crack or split and maintains surface protection against corrosion. For example, UL 6 specifically requires that the conduit exterior coating should not detach from its metal substrate after a straight conduit is bent into a 90 degree curvature. The use of prior art corrosion resistant conduit systems involves significant material and labor costs due to the complexity of the process of making conduit coated on the interior and exterior surfaces, as well as maintaining the corrosion resistant properties during field modification of the conduit (including conduit bending and fitting installation specific to each installation). The conduit coatings that are presently used on the exterior of corrosion resistant conduits are formulated to be applied in a bath, and also to be removed during the threading process. Due to limitations of available coating compounds, the resultant conduit outer coating is compliant, and prone to abrasion.
One difficulty with prior art coated conduits and fittings stems from threading each end of the conduit. This is the conventional corrosion resistant conduit-connection method and it increases field-labor over other conduit systems due to additional steps required to maintain corrosion resistance at this critical interface. During the cutting and threading of coated conduits, special attention is required in order to maintain the integrity of the polymer coating. This increase the installation time and the cost of the coated conduit system over that of a standard uncoated conduit system. Furthermore, tightening of the connections imparts forces on the conduit, fittings, and/or conduit bodies, which can damage the coatings. Accordingly, there is a need for a corrosion resistant electrical conduit system that can use push-fit connectors, which reduces (if not eliminates) torsional moments and stresses to the polymer coatings, with the added benefit of reduced installation time and efforts and increased reliability of the overall electrical distribution system.
Other corrosion resistant conduit systems of nonmetallic materials such as PVC and fiberglass do not offer the strength, stiffness and impact resistance of metallic based conduit systems. These systems also require hot boxes to effectively fabricate required custom bends during field installation. During field bending of the non-metallic conduit system, the section of conduit being modified requires heating to the point where the conduit may be easily bent, and then the conduit held in that position until the conduit sufficiently cools. As such, significant time is required to fabricate even the simplest field bend of PVC or fiberglass type conduits.
In order for metallic conduit to perform effectively as equipment grounding conductors, it is crucial that it is installed properly with tight joints. If a fault occurs, proper installation ensures a continuous, low impedance path back to the overcurrent protective device. If joints are not made up tightly or if there is a break in the ground fault current path under fault conditions, there is a possibility of electric shock for anyone (or anything) who comes in contact with the conduit system. Therefore, the NEC® requires all metal enclosures for conductors to be metallically joined together into a continuous electrical conductor connected to all boxes, fittings, and cabinets so as to provide effective electrical continuity. Polymer coated electrical conduit systems must comply with the same requirements as uncoated steel conduit systems and provide electrical continuity between coated conduits and coated conduit fittings. Accordingly, there is a need for a coated conduit system that can be easily constructed and forms a continuous electrical conductor system.